1. Field of the Invention
The invention relates to a reactive rectification column, preferably for conversion of methylmercaptopropionaldehyde prepared from crude acrolein and crude methyl mercaptan to 2-hydroxy-4-methylmercaptobutyric acid and/or methionine, and to the use thereof in a method for the production of 2-hydroxy-4-methylmercaptobutyric acid and/or methionine.
2. Discussion of the Background
For the synthesis of acrolein, several routes are conceivable in principle. The background art utilizes the condensation of formaldehyde and acetaldehyde, or else the dehydration of glycerol. A further customary process which is dominant on the industrial scale is the partial oxidation of propene. For preparation of acrolein, the reaction is performed in a conventional process, for example in a shell-and-tube reactor filled with a catalyst. The heterogeneously catalysed oxidation of propene forms not only the acrolein target product but also further secondary components, principally acrylic acid and carbon oxides (CO, CO2). Further secondary components are in particular acetaldehyde, formaldehyde, propionaldehyde, unsaturated organic acids, ketones and water (see U.S. Pat. No. 6,057,481 and DE 1 618 889). The reaction gas comprising the acrolein is therefore first scrubbed with water or with a water-solvent mixture in order to free it of high-boiling compounds such as acrylic acid and acetic acid, and polymer residues. Introduction of the cleaned reaction gas into cold water gives an aqueous acrolein solution. Uncondensable gases such as N2, excess propene and any propane (according to the propene quality), CO and CO2 leave the absorber via the top and can in some cases be used as an inertization medium in the reaction section. The remaining components of the offgas are sent to an incineration plant. The crude acrolein purified to free it of high-boiling compounds and uncondensable gases is obtained from the aqueous acrolein solution under reduced pressure and at elevated temperatures, and suitable stabilizers, for example hydroquinone, are added to counteract polymerization.
The background art specifies acetaldehyde and water as secondary components which are formed in the preparation of acrolein and are present in the crude acrolein. Distillation of the crude acrolein is possible, but crude acrolein purified by distillation would still contain traces of acetaldehyde. If the crude acrolein were then to be reacted with methyl mercaptan (MC) to give 3-methylmercaptopropionaldehyde (MMP), the background art (U.S. Pat. No. 6,057,481, DE 1 618 889) states that acetaldehyde (and other volatile secondary components) are removed from the crude acrolein advantageously only after it is converted to MMP. The reason for this is that the volatile acrolein is then converted to a high-boiling component and can thus be separated more easily from acetaldehyde or low boilers in general, and these components do not adversely affect the synthesis of 3-methyl-mercaptopropionaldehyde from acrolein and methyl mercaptan. The same applies, as already described in DE 103 59 636 A1, to the use of crude methyl mercaptan from which, after reaction with acrolein to give 3-methylmercaptopropionaldehyde, the unconverted H2S and methanol reactants can be removed more easily.
It has additionally been stated that the crude acrolein, in addition to the known compounds such as acetaldehyde, may also contain traces of allyl alcohol (see Ullmann's Enzyklopädie der technischen Chemie, Weinheim 2007, “Acrolein and Methacrolein” chapter, section 4, page 10). Studies by the inventors have now found that further unsaturated compounds such as allyl acrylate, allyl acetate and benzaldehyde may likewise be present, which are obtained as by-products of the partial oxidation of propene over a heterogeneous catalyst. The allyl components, especially allyl alcohol, even after the reaction of the crude acrolein with methyl mercaptan in the presence of a catalyst to give 3-methylmercaptopropionaldehyde and the subsequent workup (see DE 103 596 36 A1), are not completely removed and can accumulate in downstream processes, especially in the conversion of 3-methylmercaptopropionaldehyde via methylmercaptopropionaldehyde cyanohydrins to methionine (or else to 2-hydroxy-4-methylmercaptobutyric acid (MHA)). In Example 24 of U.S. Pat. No. 5,905,171, it is likewise stated that, after reaction of acrolein which originates from the catalytic oxidation of propene with MC to give MMP, allyl alcohol is present in the final product. However, it is not stated how this component is removed again from the product in the downstream processes to give methionine and/or MHA or discharged from the process.
Studies by the inventors have confirmed that the accumulation, especially of allyl alcohol, occurs in the top section of the column in which the alkaline hydrolysis (e.g. EP 2 133 329 A2 or EP 0 780 370 A2) of 5-(2-methylmercaptoethyl)hydantoin takes place (see formula III). However, discharge of the unwanted by-products via the bottom is ruled out under the prevailing conditions, as in EP 0 780 370 A2.
However, removal of these unsaturated compounds is necessary since allylic components such as allyl alcohol basically have a higher toxicity than the corresponding saturated alkyl components thereof. The methionine and/or MHA products to be prepared from the acrolein are used as animal feed additives in modern animal nutrition. The production process therefore has to ensure that all toxic compounds are removed. From a technical point of view, accumulation of the unsaturated secondary components such as allyl alcohol additionally disrupts the downstream process steps. For example, in the process for preparing methionine, the product solution is purified after the hydrolysis over activated carbon as an adsorbent. An elevated concentration of impurities would consequently lead to an increased consumption of activated carbon and thus distinctly shorten the service life, which would lead to the production of greater amounts of waste streams and/or emissions from the plant. Due to the low concentration in the crude acrolein, a distillative separation is, however, achievable only with high capital and operating costs. Furthermore, valuable acrolein would be lost in the additional workup step, which reduces the overall yield of the process. For reasons of process reliability, a conversion of the crude acrolein and subsequent removal of the secondary components is likewise more favourable since acrolein has a much higher reactivity than the MMP conversion product. The consequence thereof is an increased plant availability since a distillation of more highly concentrated acrolein can lead to deposits in different apparatuses or pipelines, which would entail intensive purification steps and an increased number thereof.